Suspension bridges typically have two towers defining a main span therebetween and two side spans extending from each tower to respective anchorages at opposite ends of the bridge. Each of the towers supports main cables, extending from an anchorage at one end of the bridge to an anchorage at the other end of the bridge, that constitute the primary load carrying components. The main suspension cables are traditionally constructed by a mechanical process of “cable spinning” whereby spools of high strength galvanized steel wire are secured at each anchorage and spinning wheels pull the wire off the spools. The wire travels from one anchorage, up and over the towers, to the other anchorage. The cable is slidably accommodated in respective saddles mounted on the top of the towers for transfer of the tensile load on the cables to vertical compression load on the towers.
Although most cables are constructed of individual parallel wires, some cables are formed of locked coil strands or helical wires. By way of example, a typical wire diameter, including a zinc coating, is approximately 0.196 in. and has an ultimate strength of approximately 225 ksi. A typical cable of a major bridge generally ranges in diameter from 15 in. up to about 36 in. and consists of from about 5,000 to approximately 28,000 individual wires. The individual wires are tightly wrapped transversely by wrapping wires and restrained from lateral expansion by a radial pressure exerted by the wrapping wires. Vertical suspender or hanger “ropes”, attached to each of the main cables by cable bands, support an underlying suspended bridge deck. The distance along the main cable, between the cable bands define panel lengths commonly referred to as “panels”. In cable-stayed bridges the cables are attached directly from the towers to an underlying suspended bridge deck.
Suspension bridges, as well as cable-stayed bridges, are usually designed for a service life of 100 years or more. The cables should preferably have a service life comparable to the main structure since replacement of the cables is rarely considered feasible. The cables are however, subject to time-dependent degradation as a result of environmental factors such as hydrogen embrittlement, corrosion fatigue, stress corrosion, cracking, corrosion pitting, and other atmospheric conditions. Additionally, the dead load and the live load, over time, have a detrimental effect on cable strength.
It should therefore be apparent that bridge inspection and maintenance becomes increasingly important as a safety precaution especially as a bridge ages. Furthermore, a method to evaluate the remaining load carrying capacity of the cable and to estimate the residual life span of the cable is of critical importance.
Generally, bridge inspections are focused on visual and subjective evaluation of corrosion damage to the wire surface. Existing cable inspection techniques involve selecting sections of the cables that are judged to be most vulnerable, uncovering the wrapping wires to expose the individual cable wires, separating the cable wires by inserting wedges to allow for visual inspection of the interior wires and then removing a limited number of wires for laboratory testing. The reliability of this method is questionable since only a small portion of a very limited number of wires can be visually inspected and tested. Another shortcoming of this sampling procedure is that it generates a relatively small number of wires within the total population of wires in the cable and further it is not based on a random sampling technique and therefore may not be representative of the larger wire population from which it is drawn. Additionally, the previously known cable strength assessment methods do not always provide reliable information as to cable integrity for the reason that when the cable wires are inspected, even in new condition, there may be small or invisible and thus undetectable cracks that may reduce wire load capacity and/or corroded but unbroken wires that have unknown load capacity. Furthermore, the previously known methods assess the load carrying capacity of the cable based upon measurement of the wire ultimate strength without regard to ultimate elongation and since ultimate elongation is a factor of wire degradation, the omission of this criterion adversely affects the assessment analysis.
It should be further noted that the previous methods for determining cable load capacity also failed to introduce fracture toughness analysis for assessing the strength of cracked wires.
Therefore it should be apparent that currently available cable strength assessment procedures do not provide a well-defined and comprehensive method for cable condition assessment, and do not provide results that are consistent and not dependent on the organization that conducts the evaluation and the effectiveness of the procedure utilized by that organization.
The current technology and assessment methods are therefore of only limited value for providing an accurate evaluation of cable strength and residual cable life. The present invention represents a significant advance in the evolution of techniques for evaluating cable strength and residual cable life.